I've been running Adsense for 6 years & have never seen white space where Adsense blocks should be showing up on the page. IMO that option is outdated.

You need to login you c panel and make a folder on your domain. you can give a name of this folder like 'ads'. If your domain name is 'www.domain.com' then you can easily access that folder with 'www.domain.com/ads' . You can save your all backup ads on this folder. you need to do is creating ads in the same height and width as Adsense ads.

I got it now, I didn't realize that you could only have 3 ad units and 3 link units per page. I had them set to all ad units which made 3 of them go blank.

When you check your adsense revenue, click on "Ad Units" and it will show you what your coverage is for each type of ad you run on your site. My smaller sites always have 100% coverage which means that Google is able to provide an ad for every request they get from the site. My biggest site with tens of thousands of ad requests daily rarely has coverage less than 99.89%.

If your coverage is that high don't worry about white space being served instead of ads. At 99.89% Google is delivering 9989 ads for every 10,000 requests they get from that site. 11 blanks out of 10,000 isn't enough to worry about.

On the other hand, if you're running under 95% and you have a lot of traffic you could be losing a lot of money.

From what I'm seeing at my sites, Google has no big problem filling ad space so I'd bet most of you don't really need to worry about it.